File tree expansion/collapse behavior

Something about the expand/collapse icon feels slightly off to me. It might be because while the arrow directions make sense and mimic the behavior of the sidebar, since the button to expand/collapse isn't actually on the sidebar, the experience is slightly jarring.

@psimyn Suggested this in Slack-

or we can put the file tree in the left sidebar!

you then need back button or somethinng in the sidebar to go back to global navigation. Multi-purpose sidebar is used in Google Cloud admin interface

This would be a pretty huge change, but it is interesting. It might make better use of the space, since we currently have to add the file tree outside of the main container. @jkarthik what do you think?